Time-to-digital converters (TDCs) are digital circuits that measure the delay of a pulse signal and convert this delay directly into a digital signal. TDCs are widely used in precision time measurement instruments. TDCs may be produced using CMOS (Complementary Metal Oxide Semiconductor) processes, which are becoming increasingly prevalent in the semiconductor industry.
A time-analog-to-digital converter (TAD) is an analog-to-digital converter (ADC) that is implemented using time-to-digital conversion techniques. In one type of TAD, a variable delay is first extracted from the input signal, and this delay is then measured (digitized) by digital elements. The digital elements include a chain of delay units (DUs), which form a delay line. The DUs may be, e.g., CMOS inverters. This approach differs from conventional ADCs in that the entrance of the input signal is to a sampling stage and no analog devices are used.
In one type of TAD, the input signal is connected to the positive supply terminal, Vdd, of the inverters. A clocked pulse is applied to the gate of the first inverter in the delay line in order to obtain variable delays on the input pulse through the Vdd terminal, where the input signal is connected. After the signal dependent delay on the output pulse is obtained, the delay is digitized using a time-to-digital approach. Because no sample-and-hold circuit is needed, an all-digital design is possible. However, the input signal range to be digitized is limited due to the threshold values of devices in the delay line. Input voltages at the Vdd terminal lower than the threshold voltages of the PMOS and NMOS devices in the DUs (inverters) create a dead region in the conversion.